This proposal aims to comprehensively describe host and virus biology associated with HIV-1 subtype C (HIV-1C) mother-to-child transmission (MTCT). We hypothesize that HIV-1 C MTCT is a non-stochastic event mediated by both placenta! and viral determinants. To address this hypothesis, we will: 1) genotype and phenotype vertically transmitted envelope (env) genes, 2) characterize envelope genes isolated from archived placental biopsies, 3) describe the placental environment during MTCT. All experiments will use previously collected samples from a well-characterized cohort of 744 HIV-1 C-infected pregnant Malawian women characterized as nontransmitting (NT), in utero (IU) mother-offspring pairs (MOPs) and intrapartum (IP) MOPs. In a preliminary study, we enumerated HIV-1 C V1/V2 env diversity in 22 MOPs and found that infants had significantly fewer variants than their mothers and confirmed that HIV env diversity is restricted during MTCT. During the mentored portion of this grant (Specific Aim 1), I will test the hypothesis that restriction is facilitated by specific env genotypes. This will be accomplished by cloning full-length envelope genes to: a) compare V1/V2 loop lengths, b) enumerate putative glycosylation sites, c) pseudotype the cloned env genes against CCR5, CxCR4, CD4hi and CD4lo cells. As an independent investigator, I will focus on ID MTCT and test the hypothesis that the placenta is a unique HIV-1 C compartment. Using a heteroduplex tracking assay, I will compare HIV-1 C env genes from matched peripheral blood, placental blood, placental biopsies, and cord blood (Specific Aim 2). Placental specific variants will be characterized using the methods acquired during the mentored phase. In Specific Aim 3, transmitted and placental specific variants will be psuedotyped against primary placental trophoblasts and Hofbauer cells. Finally, in Specific Aim 4, placental biopsies will be analyzed for HIV-1 C, CCR5, CD4, and CXCR4 expression. The results of this project will provide a comprehensive description of the HIV-1 C envelope genes and the placental environment conducive to in utero HIV-1 C MTCT.